Outdoor Paint Calculator
Figure out exactly how much outdoor paint you need before you stand in the Home Depot aisle guessing. Enter your exterior wall area, siding type, and number of coats, and this outdoor paint calculator returns the gallons to buy plus a cost estimate for the paint and optional primer. The result adjusts for siding texture because rough-sawn wood and stucco soak up noticeably more paint than smooth fiber cement.
How the Outdoor Paint Calculator Works
The formula is straightforward: paintable area (total walls minus doors and windows) multiplied by the number of coats, then divided by effective coverage per gallon. A gallon of exterior latex covers about 300 square feet on smooth surfaces. That number drops on rougher substrates.
Exterior paint coverage by siding type:
| Siding | Effective coverage per gallon |
|---|---|
| Smooth (fiber cement, sealed stucco) | 300 sq ft |
| Lap siding (vinyl, wood lap) | 273 sq ft |
| Rough sawn wood / T1-11 | 214 sq ft |
| Stucco / textured masonry | 231 sq ft |
| Brick / CMU | 222 sq ft |
Estimating Exterior Wall Area
If you don't have a measured number, estimate by taking your home's perimeter (the distance around the outside walls) and multiplying by the wall height. A 40 by 30 foot single-story house has a perimeter of 140 ft. At 9 ft from foundation to eave, that's 1,260 sq ft of siding before subtracting doors and windows.
For two-story homes, use 18-20 ft of wall height. A 40 by 30 two-story has roughly 2,520-2,800 sq ft of exterior wall. Subtract about 21 sq ft per exterior door and 15 sq ft per standard window.
Paint Grade Matters Outdoors More Than Indoors
Premium exterior paint (Sherwin-Williams Duration, Benjamin Moore Aura Exterior) carries a 25-year warranty and genuinely outlasts economy paint by 2-3x. The cost difference on a whole-house job is typically $300-500, which is cheap insurance against repainting in five years. Economy paints are fine for sheds, fences, or rental properties where you expect to repaint soon anyway.
When to Prime
Always prime bare wood, severely faded old paint, or when switching from dark to light colors. Skip primer if you're applying the same color over sound existing paint. Exterior primer covers about 275 sq ft/gallon.
Best Time to Paint Outdoors
Paint when temperatures are 50-85 degrees F with low humidity and no rain forecast for 24 hours. Avoid painting in direct sun on hot days because the paint can skin over before it bonds properly. Spring and early fall are the most reliable windows in most regions.